A charged particle detector is an indispensable part of a charged particle (ion or electron beam) instruments, for example, a scanning electron microscope (SEM). In a SEM, an electron beam emanated from an electron source is focused into a fine probe over a specimen surface and scanned by a deflection unit in a raster fashion; and signal electrons released from the specimen, including secondary electrons and back scattered electrons, are collected by charged particle detectors and the signal intensity is converted into the gray level of an image pixel corresponding to the location of the electron probe on the specimen surface. Scanning of the electron probe will then form a gray level mapping thus producing an image of the specimen surface.
There common used detectors in SEM are scintillator-photomultiplier tube (PMT) combination type (e.g. an Everhart-Thornley detector), semiconductor type, and microchannel plate type. The scintillator-PMT type, due to their high gain and low noise properties in respect to the semiconductor type and microchannel plate type, is more frequently used in a high resolution and low beam current SEM. Traditionally, this type of detector is consisting of a light guide, a scintillator disc is attached to the front face of the light guide, and the rare end of the light guide is coupled to a photomultiplier tube. Secondary electrons and backscattered electrons emit from sample surface impinged on scintillator disc and, in response, generate light signals. The light guide collects the light signal and directs it to PMT. In a conventional design, the electrons to light signal conversion efficiency and light signal collection are low. In order to compose an image with enough brightness and contrast, a large magnification PMT or magnifying circuit is needed, which will introduce a larger electric noise into the image. Since the electron to light conversion efficiency is depending on the chosen scintillator material, thereby, it is expected to improve efficiency of the light collection before the PMT.
FIG. 1 illustrates a typical SEM system with a prior art electron detection device that is positioned above objective lens. The SEM configure with an electron source 101, a gun lens 102, and objective lens 103. Primary electron beam 112 generate from electron source 101 moving along the optical axis 111 through the center hole 107 of the detection device 105 strike sample 104 surface. Backscattered and secondary electrons 106 caught by scintillator disc 108, are converted to light sight signal and are directed through light guide 109 to a PMT 110.
There were many scientists put their efforts on signal collecting efficiency improvement. For example, U.S. Pat. No. 6,211,525 by Cowham, described an electron-gathering light guide for collecting backscattered electrons, wherein the electron receptor has a concaved working surface. Cowham concave the electrons collecting surface design to improve the receiving efficiency.
U.S. Pat. No. 6,768,836 and U.S. Pat. No. 6,775,452 by Howells, waveguide of optical glass (reflective index=1.5) is re-designed so that backscattered and secondary electrons are detected by a phosphor deposited waveguide. All waveguide surfaces are polished to specularly reflect light impinging on surface.
U.S. Pat. No. 7,659,514 by Adamec, Adamec's design shape the scintillator disc surface to an ellipse or a parabola and set the opening (where primary electron beam passing through) on the focus of the ellipse or the parabola to enhance the light signal collection efficiency.
U.S. Pat. No. 6,545,277 by Kella et al., proposed a tent-like shape grid to compensate a “shadow” area created by the primary beam passing light-guide within the scintillator in order to enhance the detection efficiency of the secondary electrons.
U.S. Pat. No. 6,031,230 by Toumashu, provided a design for an SEM device which allows an optical microscope used to locate the specimen's position to have a large viewing angle and also enables the detection surface of the X-ray detector of the element analyzing device to have as wide solid angle and a viewing angle as possible with respect to the observation point on the specimen; and which allows the reflected electron detector to receive a large amount of reflective electrons from the specimen.
All these prior arts that mentioned above were emphasized on redesign the shape of scintillator disc to enhance light signal generating efficiency. The following discussion describes the disadvantages of conventional design. FIG. 2 illustrates an example of prior art electron detection device that the light guide is over the top of scintillator disc. An assembly shown in FIG. 2A for a detector includes a scintillator disc 201, a light guide 202 made of optical glass and a metal tube 203 to permit charged particle or for example electron beam to passing through. Backscattered and secondary electrons emit from specimen surface (not shown) strike the scintillator disc 201 and the impact point become a photon source that generating light signals or photons. The produced photons scatter to all directions from the impact point. Only photons, however for example, have a motion component moves toward right have chance to enter light guide 202 and thereafter be collected by PMT 208. Photons will loss part of its energy when crossing the boundary from one medium to another due to reflection at the boundary (Fresnel Loss). Only when the photon has a reflection angle greater than the critical angle of reflection of the boundary can perform total reflection without transmission to another medium and without energy loss. For example, the Fresnel Loss between air and most plastics and glass is about 4% at each boundary crossing; the critical angle of reflection between air and most plastics and glass is approximately 42°. Generally speaking, less than 50% of the scintillator produced photons can reach the PMT 208. Most of the produced photons are wasted, be adsorbed by scintillate disc and light guide, or transmit themselves away on the way to the PMT 208.
In FIG. 2B, 2 photons paths 204, 205 are indicated. The path 205 represents photons from the impact points that have component moving toward right. These photon arrive the front of PMT after several reflections along the light guide 202. In each reflection of the zigzag path toward PMT, the photon loss part of its energy due to different refractive index between light guide and air. As the FIG. 2B shows, due to the Fresnel Loss, the photon path 205 split into path 206 (refraction into air) and path 207 (reflection back to light guide), but only the reflected part 207 can be received by PMT. The path 204 represents photons from the impact points that have component moving toward left. The photons are moving toward the opposite side of the PMT along the light guide 202 will disperse into surrounding or be adsorbed by the light guide itself and have little contribution to light signal collection. To save the light signals which move to the opposite direction of the PMT, the light guide is usually coated with reflective material such as aluminum and the surface of light guide is polished to enhance reflection on the light guide surface.
FIG. 3 shows another example of prior art that light guide is connected to scintillator disc in series formation. An assembly shown in FIG. 3A for a detector comprises a scintillator disc 301, a light guide 302, a metal tube 303 for charged particle (i.e. electron) beam pass through, and a PMT 313. Back scattered and secondary electrons emit from specimen surface (not shown) strike the scintillator disc 301 the impact point become a photon source. The produced photons scatter to all directions from the impact point. Only photons, however for example, have a component move toward right have chance enter light guide 302 and thereafter be collected by PMT 313. The scintillator disc 301 is shaped as an ellipse and the metal tube 303 is located on one of the focus of the ellipse. The photons from the impact points that have component moving toward right, represents by path 305 of FIG. 3C, have chance to reach PMT 313 after tortuous reflection path within the light guide 302. The path 304 represents photons from the impact points that have component moving toward left. The photons moving to the left will finally be reflected back by the ellipse shaped surface, passing through the other focus of the ellipse and moving toward right as the photon path 305. Photons will loss its portion of energy in each reflection with an incident angle less than the critical angle due to the Fresnel Loss. Therefore photon's energy decay rapidly due to the tortuous path within scintillator disc 301 and light guide 302 before photon reach the PMT 313 as shown in FIG. 3C and FIG. 3D.
The present invention propose a new design of detection device including the shape of light guide and the shape of scintillator disc to enhance photons collecting efficiency.